add image scanning on pull_request

Signed-off-by: zhzhuang-zju <[email protected]>

.github/workflows/ci-image-scanning.yaml

@@ -1,6 +1,7 @@
 name: image-scanning
 on:
-  push: 
+  push:
+  pull_request:
 jobs:
   use-trivy-to-scan-image:
     name: image-scanning
@@ -38,13 +39,15 @@ jobs:
           vuln-type: 'os,library'
           output: 'trivy-results.sarif'
       - name: display scan results
+        if: ${{ github.event_name == 'pull_request' }}
         uses: aquasecurity/[email protected]
         with:
           image-ref: 'docker.io/karmada/${{ matrix.target }}:latest'
           format: 'table'
           ignore-unfixed: true
           vuln-type: 'os,library'
       - name: Upload Trivy scan results to GitHub Security tab
+        if: ${{ github.event_name == 'push' }}
         uses: github/codeql-action/upload-sarif@v2
         with:
           sarif_file: 'trivy-results.sarif'